(NBC NEWS) More women are choosing to live with men first without marriage and more of those relationships are lasting longer, according to a new government study that tracks the continuing climb of cohabitation in the U.S.

Nearly half of women in what researchers call “first unions” with men — 48 percent — moved in with no wedding vows according to interviews conducted between 2006 and 2010, up from 43 percent in 2002 and 34 percent in 1995.

About 40 percent of those relationships became marriages after three years, but about 32 percent continued as cohabitations, suggesting an important new role for the arrangement once known as “shacking up,” says one researcher.